1. Field of the Invention
The present invention relates to a semiconductor device having a junction type field-effect transistor (referred to as a junction type FET hereinafter).
2. Description of the Prior Art
A plan view of a prior-art semiconductor device having a conventional junction type FET is shown in FIG. 6(A), and an example of its circuit diagram is shown in FIG. 6(B).
Referring to FIG. 6(A), a source electrode (S) 2 and a drain electrode (D) 3 are formed on a surface of a semiconductor substrate (a semiconductor chip), where the electrodes 2 and 3 are composed respectively of bonding pads 21 and 23, and pluralities of finger parts 22 and 24 which extend respectively from the bonding pads 21 and 23 to the section above an N-type active region to be connected respectively to N.sup.+ -type diffused layers formed in that section. Further, P.sup.+ -type diffused layers are formed as a gate region (G) between the N.sup.+ -type diffused layers, and a gate electrode is led out from a P.sup.+ -type rear surface of the semiconductor substrate.
In the semiconductor device having the conventional junction type FET, a diode (Di) is formed between the gate electrode (G) and the source electrode (S) on the input side, and a Zener diode (Z-Di), for protecting the junction type FET against an overvoltage which is applied to the drain electrode, is formed between the drain electrode (D) and the source electrode (S) on the output side, as shown in FIG. 6(B) though omitted from FIG. 6(A). Such a technique is disclosed in, for example, Japanese Utility Model Application Laid-Open No. Hei 2-56438 (1990).
Since, however, the capacity (output capacity) between the source electrode and the drain electrode of the conventional junction type FET is a small parasitic capacity of the order of 2 to 3 pF, it is difficult to satisfactorily isolate high frequency noises from the outside. This capacity is computed by C.sub.GD (parasitic capacity between the gate electrode and the drain electrode)+C.sub.DS (parasitic capacity between the drain electrode and the source electrode). Since one has C.sub.DS .apprxeq.0, to obtain a large value for the capacity it is necessary to make C.sub.GD large by increasing the area of the gate region of the P.sup.+ -type diffused layer. However, an increase in the area of the gate region results in the deterioration in the characteristics, for example, a value of the conductance g.sub.m of the junction type FET is decreasing.
On the other hand, obtaining a sufficiently large capacity between the source electrode and the drain electrode by means of the Zener diode for the protective circuit requires a very large area, which is unrealistic.
Moreover, since these capacities are P-N junction capacities, a magnitude of the capacity depends on the difference between potentials applied to the source electrode and the drain electrode so that an isolation performance against the high frequency noises from external circuits varies with the output level of the junction type FET.
Additionally, it is not possible to effectively isolate the high frequency noises even if a capacitive element is installed outside the semiconductor substrate (semiconductor chip) because the capacitive element connected between the source electrode and drain electrode is not provided nearby the junction type FET.
As described in the above, in the conventional junction type FET it is not possible to isolate noises from external circuits so that there is a problem in that the characteristics (mutual conductance and the like) of the junction type FET is not stabilizing.
In particular, when the junction type FET is incorporated in an electret capacitor microphone (miniaturized microphone used in a portable telephone set such as a land mobile radio telephone set or in a portable radio equipment) as an amplifier, there arises a problem that it is liable to pick up noises of frequency higher than 10 kHz under an influence of a high frequency oscillator which generates a carrier wave nearby the junction type FET.
On the other hand, in the electret capacitor microphone or the like, it is necessary to discharge the electric charge, charged between electrodes, quickly so that the adoption of the junction type FET having an input gate of diode construction is required and it is not possible to adopt a MOSFET having a large input capacity, namely, having an input gate of capacitor construction, because of the deterioration of a speed corresponding to an input signal.